Ex vivo production of platelets and megakaryocytes (Mk) offers solutions to the major clinical problems of donor platelet shortages and scarcity of HLA-matched products. Multiple scientific breakthroughs have paved the way toward this goal. The principal remaining roadblock consists of an intrinsic barrier to scalability. Highly proliferative fetal-type Mk progenitors yield relatively few and hypofunctional platelets due to impaired morphogenesis (e.g. enlargement and polyploidization); adult-type Mk yield more abundant and functional platelets but have minimal proliferative capacity. An ability to safely circumvent these limitations, by combining progenitor expandability with efficient platelet production, will be critical for cost-effective scale-up. Accomplishment of this goal requires a detailed understanding of the molecular mechanisms underlying the ontogenic switch, i.e. the transition from fetal to adult Mk morphogenesis. Control over this switch will enable efficient scale-up by exploiting in a sequential manner the proliferative capability of fetal Mk followed by the thrombopoietic potential of adult Mk. Our lab recently discovered a molecular basis for the Mk ontogenic switch (Elagib et al. J. Clin. Invest., 2017). Specifically, an RNA-binding factor IGF2BP3 functions as a fetal- specific master regulator by suppressing expression of the transcription factor MKL1, which orchestrates the cytoskeletal remodeling of adult-type Mk. Pharmacologic repression of IGF2BP3 with BET inhibitors induced MKL1 expression and adult morphogenesis but also caused growth arrest, compromising polyploidization. In this proposal, we identify an alternative, improved approach of circumventing IGF2BP3 repression by promoting nuclear translocation of MKL1. To accomplish this strategy, we have targeted Dyrk kinase activity, which has been implicated in cytoplasmic retention of MKL1 and in Mk abnormalities in Down syndrome. Pharmacologic Dyrk inhibition strongly enhanced cord blood Mk morphogenesis, ex vivo platelet release, and in vivo platelet production in xenotransplanted immunodeficient mice. This approach also strongly enhanced morphogenesis of iPSC (induced pluripotent stem cell derived) Mk, which normally have an early fetal phenotype. Mechanistic studies using knockout mice and knockdowns in human progenitors support a critical role for MKL1 regulation, mediated by Dyrk1a phosphorylation of Ablim2, an actin regulatory factor. The critical influence of physical milieu, e.g. stiffness and shear, on Mk morphogenesis has been attributed to MKL1 activation. Our results suggest that Dyrk1a inhibition provides a direct, potent, and tunable stimulus for Mk morphogenesis that bypasses specialized culture requirements. This approach could thus obviate cost and safety issues associated with specialized mechano-bioreactors. The proposed experiments will determine key steps in Dyrk kinase control of MKL1 in iPSC and cord blood Mk, to permit optimal design of systems with inducible Mk morphogenesis and platelet production. In addition, clinically feasible strategies for targeting this pathway will be rigorously tested for Mk morphogenesis and platelet production in cord blood progenitors.